Red-emitting phosphors that are excited by light corresponding to near-UV to blue light-emitting diodes (LEDs) are needed to improve the color rendering properties of white LEDs or, when white LEDs are used as the backlights in liquid crystal displays, to enhance color reproducibility, and are being been studied for this purpose. In particular, JP-A 2009-528429 (Patent Document 1) mentions that complex fluoride phosphors obtained by adding manganese to a complex fluoride of the formula A2MF6 (wherein A is, for example, Na, K or Rb, and M is, for example, Si, Ge or Ti) are useful.
Patent Document 1 discloses one method for producing such phosphors wherein a hydrofluoric acid solution in which all of the constituent elements have been dissolved or dispersed is concentrated by evaporation, inducing precipitation (evaporative concentration method). Another production process, disclosed in U.S. Pat. No. 3,576,756 (Patent Document 2), is a method wherein hydrofluoric acid solutions in which the respective constituent elements have been separately dissolved are mixed together, following which acetone (a water-soluble organic solvent) is added, lowering the solubility and thereby inducing precipitation (poor solvent addition method). In addition, JP No. 4582259 (Patent Document 3) and JP-A No. 2012-224536 (Patent Document 4) disclose methods wherein the element M and the element A in the above formula are each dissolved in separate hydrofluoric acid-containing solutions, manganese is added to either one of these solutions, and the solutions are mixed together, thereby inducing precipitation of the phosphor (mixing and precipitation method).
The processes for producing the known manganese-doped complex fluoride phosphors of the formula A2MF6 (wherein A is, for example, Na, K or Rb; and M is, for example, Si, Ge or Ti) mentioned above can be applied to small-volume synthesis in the laboratory. However, carrying out these processes industrially on a large scale will require further investigation. Even in the case of methods, among those already known, which are based on the mixture of aqueous solutions (Patent Documents 3 and 4) and are thought to be highly suitable for mass production, production operations with actual industrial production equipment in mind are still under investigation.
In particular, in the evaporative concentration method, the product purity is inadequate, and in the poor solvent addition method, fine particles having a poor crystallinity tend to form. For these and other reasons, phosphors produced by either of these methods have lacked satisfactory emission characteristics. For these reasons, the mixing and precipitation method appears to be a more improved method, although the detailed conditions are still under investigation. There are cases in which, depending on the concentration and composition of the reaction liquid in particular, sufficient characteristics cannot be obtained, or in which, for reasons having to do with solubility, precipitation itself does not arise.
Also, in recent years there has been a technical call for increasing the luminous flux per white LED device, and the amount of current passed through LED chips is being increased for this purpose. However, when this is done, the temperature of the phosphor used ends up rising, as a result of which the phenomenon of decreased light emission by the phosphor may occur. Phosphors of poor crystallinity that are produced by the conventional methods described above undergo a particularly large decrease in emission characteristics at such elevated temperatures, prompting a desire for improvement.